The present invention relates to a device reading in a reducing a document in a reduced size which is suited to produced reduced copies of a relatively wide document, e.g. display board of a graphic input device.
An optical arrangement for a document reading device is available in various configurations. For example, there is known an arrangement including a first mirror which has a reflective surface located to face in an inclined position a glass platen adapted to load a document, a second mirror having a reflective surface which is perpendicular to the glass platen and serving to reflect light from the first mirror, a third mirror having a reflective surface which faces that of the second mirror, a lens mounted on the reflective surface of the third mirror, and a fourth mirror having a reflective surface which faces in an inclined position the surface of a photoconductive element or that of a photosensitive sheet. In this kind of arrangement, the photoconductive element is exposed imagewise to the light from the document on the glass platen by way of the first mirror, second mirror, lens, third mirror, lens, second mirror, and fourth mirror. The second mirror which reflects the light twice is directed to shortening the total length of the optical path defined in the arrangement.
A problem is encountered when it is desired to copy in a reduced size a plan of A0 or A1 size, a white board of a graphic input device or so-called electronic blackboard, and other relatively wide documents, because such documents have to be imaged with a considerable reduction ratio. For example, assume a condition wherein a document which is 27 inches (686 millimeters) wide is to be read by an optical system of the type using a lens having a focal length f of 30 millimeters and a charge coupled device (CCD) having 2048 pixels in total, which are arranged at intervals of 14 microns. In such a condition, the reduction ratio m is expressed as (2048.times.14.times.10.sup.-3)/686=28.672/686 and, therefore, approximately 1/23.93 (nearly equal to 4.18%). Hence, the distance from the document surface to the CCD imaging surface is as long as f(1+m).sup.2 /m =779 millimeters, resulting in the need for a long optical path. It follows that the optical path cannot be shortened even with the previously stated type of optical arrangement used as such a reducing optical system.
In relation to a reducing and reading optical system as described above, an approach which may be contemplated to cut down the size of the optical system is positioning a pair of flat mirrors in parallel and causing them to reflect a beam a plurality of times therebetween. Such a parallel mirror type scheme, however, requires a great number of structural elements, an extra space for the installation of such structural elements, and the adjustment of the angular positions of the mirrors. It is difficult for the mirrors to be mounted with their flatness maintained. In any cases, compacting the configuration of the entire optical system is impractical.